Reverse osmosis and nanofiltration membranes, particularly interfacially polymerized polyamide thin-film composite membranes, are typically prepared from polyamine monomer and polyfunctional acyl halide monomer; see, for example, U.S. Pat. No. 4,277,344. Such membranes are useful for the separation of dissolved substances, e.g., salts, from solvents, e.g., water. Thus, potable or less salty water can be obtained from seawater, contaminated water, brackish water or brine by reverse osmosis. For practical applications, the flux, i.e., the flow rate across the membrane, should exceed about 5.9 liters/m.sup.2 -hour [10 gallons/ft.sup.2 -day (gfd)] and the salt rejection should exceed about 98 percent.
Reverse osmosis and nanofiltration membranes now receiving widespread commercial endorsement are obtained by interfacially condensing a polyfunctional acyl halide having a low aqueous solubility, viz., trimesoyl chloride, with a polyamine having a high water solubility, viz., m-phenylene diamine. In general, the aqueous solution of m-phenylene diamine is absorbed on a porous support and is interfacially contacted with a nonpolar organic liquid solution of trimesoyl chloride; see, for example, U.S. Pat. No. 4,277,344. In certain nanofiltration membranes, the m-phenylene diamine is replaced with piperazine; see for example, U.S. Pat. Nos. 4,769,148 and 4,859,384.
Because of its inertness, volatility, low toxicity and non-flammability, FREON.TM. 113 or 1,2,2-trichloro-1,1,2-trifluoroethane has traditionally been preferred as the nonpolar organic solvent for the trimesoyl chloride. Recently, environmental concerns related to ozone depletion have provided the impetus to seek replacements for chlorofluorocarbons like FREON 113. Lower molecular weight hydrocarbons, such as hexane, are adequate substitute solvents with respect to inertness, volatility and toxicity, but low flash points and high flammability make hydrocarbons with fewer than 8 carbon atoms less than ideal replacements. Higher boiling hydrocarbons such as C.sub.8 -C.sub.14 alkanes, on the other hand, while having more favorable flashpoints, are less volatile and require higher drying temperatures to be effectively removed from the membrane. When exposed to drying temperatures in excess of 60.degree. C., reverse osmosis membranes suffer from serious reductions in flux and sometimes reductions in salt rejection. To prevent the loss of flux or salt rejection associated with heating at elevated temperatures, U.S. Pat. No. 5,368,889 suggests the use of a physical means not requiring a phase change for removing higher boiling hydrocarbon solvents, e.g., a water knife, an air knife, a roller or a rubber blade.
Alternatively, U.S. Pat. Nos. 4,872,984 and 4,948,507 disclose the use of monomeric tertiary and quaternary amine salts such as triethylamine camphor sulfonic acid to avoid the loss of flux when a composite polyamide thin film is heated to remove higher boiling hydrocarbon solvents. The amine salts are incorporated as necessary ingredients of the aqueous solution of polyamine which coats the porous support prior to interfacial contact with the nonpolar organic solution of polyfunctional acyl halide.
U.S. Pat. No. 4,983,291 discloses a method of maintaining high flux in dried reverse osmosis membranes by treating the membrane after formation and before drying with a solution of an acid or an amine salt of an acid, e.g., acetic acid and m-phenylene diamine, at a pH of less than 7.
While treatment of reverse osmosis membranes with amine salts, particularly tertiary and quaternary amine salts, effectively preserves the flux and salt rejection attributes of membranes subjected to drying at the relatively high temperatures required for removal of C.sub.8 to C.sub.14 hydrocarbon solvents, the process does suffer from several drawbacks. Amine salts, particularly those of tertiary and quaternary amines, are relatively expensive. Furthermore, treatment of waste streams containing quaternary and tertiary amine salts is usually difficult. Finally, quaternary and tertiary organic amines are often toxic and membranes treated with such materials need to be thoroughly washed out before being used in potable water or food contact applications. Therefore, it would be desirable to have a method of preserving flux and salt rejection in oven-dried reverse osmosis membranes which involves incorporation of an additive which is less expensive and less toxic and whose waste is more easily treatable.